An optical space transmission system for transmitting an optical signal between an optical transmitter and an optical receiver via a free space has been in practical use. However, there is a gradual trend of increasing the capacity of a signal and such a signal is to be transmitted with high speed, and thus, a high speed transmission and large capacity are also required for an optical space transmission system. Signals to be transmitted include data, image, and sound, and, in some cases, these different signals are simultaneously transmitted. In order to adapt the requests for high-speed transmission, large capacity, and simultaneous transmission of a plurality of signals, a space transmission system for wavelength-multiplexed light has been proposed; the space transmission system being for wavelength multiplexing optical signals having a plurality of wavelengths and for transmitting the wavelength-multiplexed optical signal.
There exists a conventional wavelength multiplexing optical space transmission system in which optical signals having wavelengths different from each other are multiplexed by using a plurality of dichroic mirrors in an optical transmitter of the system (for example, see Patent Document 1). FIG. 19 shows a configuration of a conventional wavelength multiplexing optical space transmitter described in Patent Document 1.
In FIG. 19, light emitting elements 52a to 52d send out optical signals having wavelengths different from each other. A dichroic mirror section 54a has characteristics of allowing to pass through a light having wavelength λ1 and reflecting a light having wavelength λ2 by a reflection surface placed in the middle thereof. Accordingly, the light having wavelength λ1 sent from the light emitting element 52a and the light having wavelength λ2 sent from the light emitting element 52b are thereby multiplexed. Also, a dichroic mirror section 54b has characteristics of allowing to pass through a light having wavelength λ1 or λ2 and reflecting a light having wavelength λ3 by a reflection surface placed in the middle thereof. Accordingly, the light having wavelength λ1 and the light having wavelengths λ2 respectively sent from the light emitting elements 52a and 52b and the light having wavelength λ3 sent from the light emitting element 52c are thereby multiplexed. Through sequentially multiplexing light as described above, a wavelength-multiplexed optical signal whose optical axes therein are aligned is generated.
On the other hand, there exists a conventional optical space transmitter in which safety of a transmission light is enhanced by using a diffusion plate (for example, see Patent Document 2). FIG. 20 shows a conventional optical space transmitter described in Patent Document 2.
In the apparatus shown in FIG. 20, a transmission light sent from a light source 61 is converted into a non-convergent type diffuse light by a reflective diffusion plate 64, thereby enhancing safety when a human body, especially the eyes, are exposed to an optical signal transmitted via a free space.                [Patent Document 1] Japanese Laid-Open Patent Publication No. 63-151230 (pages 6 to 8, FIG. 1)        [Patent Document 2] Japanese Laid-Open Utility Model Publication No. 62-58938 (page 1, FIG. 2)        
However, in the conventional configuration shown in FIG. 19, it is necessary to have a plurality of dichroic mirror sections 54a to 54c having different characteristics, leading to an increase in types and the number of constituent parts, which in turn causes difficulties in downsizing and cost reduction. Also, a reflection direction of light changes according to an incident angle thereof for each of the dichroic mirror sections 54a to 54c. Therefore, when directions of light entering the dichroic mirror sections 54a to 54c differ from each other or orientations of the dichroic mirror sections 54a to 54c differ from each other, optical axes in a signal obtained after multiplexing are displaced out of alignment. Accordingly, a highly precise angle adjustment for optical axes is required to align optical axes of a plurality of optical signals, causing problems of an increase in man-hours for manufacture and cost.
The conventional configuration shown in FIG. 20 is for enhancing safety in a case where a human body, especially the eyes, are exposed to an optical signal transmitted via a free space, and not intended to achieve a high speed transmission in the optical space transmission system by wavelength multiplexing. Also, in order to enhance safety in the conventional configuration shown in FIG. 19, it is necessary to further include a configuration such as shown in FIG. 20.
The present invention is made to solve the above-described conventional problems, and an object thereof is to provide an optical space transmitter and an optical space transmission method, for multiplexed optical light, capable of obtaining a wavelength-multiplexed signal by multiplexing optical signals having a plurality of wavelengths with a simple configuration and without requiring highly precise angle adjustment for optical axes, and for enhancing safety for a human body including, e.g., the eyes.